This invention relates to a ground enclosed, load-break switch particularly adapted for use in medium voltage power distribution systems in the range about 1 to about 36 kilovolts (kv) for interrupting currents of up to about 1 kiloampere (ka).
Load break switches used in medium voltage power distribution range circuits generally include a pair of electrodes, one being stationary and the other movable to open and close the circuit. As commonly used in three-phase systems, three or multiples of three switches are mounted in a common grounded metal enclosure. The switches are of four general types. Air break switches were the first developed. Such switches rely on air for insulation. Since air is a relatively poor insulator, relatively large clearances are required to be maintained around each switch for insulating from ground potential. Further, the contacts of the switch must be separable by a relatively large gap in order to break and isolate the circuit when the switch is opened. These switches are consequently very large and must be provided with extremely large enclosures. Therefore, air break switches are often impractical in medium power distribution systems because of the high manufacturing cost, consumption of available space, and unsightly appearance dictated by their shear bulk.
Oil insulated switches represent a significant size improvement over air break switches. Oil is both an effective insulator and arc quenching medium. This enables an oil insulated switch to be enclosed in a relatively compact grounded housing. Therefore, oil insulated switches take considerably less space than that required for a corresponding air break switch for use in the same voltage and current range. However, oil is inflammable, creating a fire hazard. Another disadvantage of an oil insulated switch is that gases generated by the arc drawn in the oil generate high pressures, posing an explosion hazard, and the accumulation of gases require special accomodations, such as venting. A further disadvantage is the danger of water contamination of the oil, which can drastically reduce the dielectric strength of the oil.
An oil switch for use in the medium voltage distribution system typically comprises a metal housing, which is grounded and filled with oil. In the typical three phase configuration, a single metal housing includes at least three switches, one for each phase, or multiples of three such switches, immersed in the oil.
Another class of load break switches employs vacuum interrupters. In these switches, the contacts are enclosed in an evacuated chamber. The vacuum environment rapidly dissipates the gaseous products of the arc drawn between the contacts of the switch to effect interruption of the current when the switch is opened. Typically, three or more such vacuum interrupters are mounted within a grounded metal enclosure. The enclosure contains an insulating medium which surrounds each vacuum interrupter. The insulating medium can be air, oil or sulfur hexafluoride.
The disadvantages and limitations of vacuum switches include the following:
1. Random breakdowns in the open position due to surface imperfections of the contacts dictate an auxilliary disconnect means that operates following current interruption.
2. The proper vacuum state of 10.sup.-5 torr is very difficult to monitor.
3. The presence of a floating ring at contact potential within the interrupter precludes practical operation of the vacuum envelope at ground potential.
4. The high frequency characteristics of the vacuum switch can cause over voltages.
5. The vacuum switch presents special problems when capacitive or inductive loads are involved.
6. There are wide statistical variations in vacuum dielectric and insulation characteristics.
Another class of load break switches for distribution systems is gas insulated switches which employ a gas for both insulation and interruption. Sulfur hexafluoride (SF.sub.6), either alone or mixed with other gases such as nitrogen, is used. Such switches can be of the gas-blast or puffer type in which the arc quenching gas is caused to flow across the contacts as the arc is formed. The relatively high velocity flow of dense gas along the arc rapidly extinguishes the arc at a natural current zero of the alternating current. In a typical three phase configuration, a grounded metal enclosure surrounds three, or multiples of three, switches. Each individual switch typically comprises an unsealed cylindrical housing of a plastic such as reinforced epoxy resin. A grounded metal housing filled with sulfur hexafluoride surrounds the interrupters with substantial clearance to prevent arcing. The SF.sub.6 within the common enclosure and within the individual switches is generally maintained at about atmospheric pressure in distribution class switches involving multiple interrupters. A higher pressure within the housing is not practical because the conventional gaskets and moving seals are subject to leakage and because the high mechanical stresses that are developed in the relatively large common housing require a prohibitively massive enclosure. Consequently, a pressure only high enough to prevent a vacuum condition at low temperatures is used, that is, 1.2-1.4 atmospheres at 60.degree. F. which is sufficient for positive pressure at a minimum temperature of -40.degree. F.
A disadvantage of each of the above described switches is that they are very heavy and bulky. Thus they require special foundations and large vaults and enclosures in which they are mounted, resulting in high installation costs, unsightly appearance, and demanding large amounts of critical space. The bulk of these switches is particularly undesirable in urban and residential areas.
Another problem with these types of switches is abrasion and mechanical wear of the contacts. The movable and stationary contacts can be solid conductors that butt against each other to close the electrical circuit. This contact configuration has the disadvantages of contact bounce and high impact stress when the switch is closed, and high contact resistance when arcing byproducts or corroded materials are trapped between the contacts. To avoid these problems, a wiping contact configuration can be used. In some designs one of the contacts has a hollow cylindrical configuration while the other is a solid cylinder which fits inside the hollow cylindrical portion of the other contact. The hollow cylindrical portion can have a solid wall configuration or can be a cluster of fingers such as that disclosed in U.S. Pat. No. 3,970,811 to Krebs. The operation of the circuit interruptor results in the surface of the contacts sliding with respect to each other. This can result in degradation of the dielectric by the wear particles from the contacts. This further reduces the effective life of the contacts, and requires expensive maintenance for repair.
Another problem with cylindrical finger contacts is the difficulty of maintaining proper axial alignment for contact engagement when the switch is closed.
An additional problem with gas filled switches is that the pressure within the enclosure can degrade to an unsafe level at which the arc developed upon opening the switch might not be quenched, resulting in rapid heating and vaporization of the contacts, and in some instances, an explosion. A pressure guage can be provided, but this does not prevent opening the switch and it constitutes an additional source of leakage.
An additional problem with sealed switches is that a malfunction within the switch can cause uncontrolled arcing, heating and consequent vaporization of metallic contacts. This increases the internal pressure of the switch and creates the safety hazard of a possible explosion of the switch.
An additional problem with distribution switches is the need for a convenient means of detecting line voltage energization of each conductor and the position of the switch contacts. Direct measurement of the high voltage levels involved is cumbersome and dangerous, and the position of enclosed contacts is not certain when the contacts are either both energized or both unenergized. For convenience, this monitoring should be possible at locations remote from the switch.
A further problem with distribution switches is the need for remote operation of the contacts. This need has been described, for example, in U.S. Pat. No. 4,187,437 to Muller, et al. The cost of remote actuation in the prior art is high because of the great energy required to operate conventional switches.
In medium voltage distribution systems, another device which can be used to interrupt current is the so-called "load break" elbow connector. Such load break elbows enjoy advantages including their small size, capability of being connected directly to a bushing and/or cable, and their submersibility, making them suitable for use in both surface and underground vault locations. However, they have important limitations and difficulties:
1. Load breaking is limited to 200 amperes and below.
2. With the load break separable elbow, a considerable personnel hazard exists, since the disconnection is drawn directly into the atmosphere and there is no shielding or protection for the operator who is required to stand in front of the device. An arc can be drawn in the open, thus creating a danger that the arc can jump to ground, thereby generating a major arcing fault of extremely high current magnitude which can be fatal to anyone in the vicinity, and destructive of any equipment.
3. Proper disconnection depends on the skill and strength of the operator using a hot stick to jerk the elbow from its connection.
4. The useful life of an elbow connector is limited to a relatively few operations since erosion of the mating parts by arcing can greatly reduce the effectiveness of the device to interrupt currents. There is some danger associated with such degradation.
5. Load break elbows are limited to single phase operation and where used on a three phase system there is necessarily a relatively long time interval between the opening or closing of the individual phases. This can cause in some cases the phenomenon of ferro-resonance with serious overvoltage consequences. In many parts of the world, only three phase switches are acceptable.
6. The right angle bend introduced by the elbow triggers a requirement that a full cable loop be included within the enclosure. These cable loops in distribution lines are extremely large, dominating the required space within the enclosure.
7. After prolonged connection, an elbow connector can stick to the mating configuration, resulting in an inordinate force being required for disconnection.
As a consequence, the use of load break elbows for load switching or load transfer is frequently restricted. These elbow connections are generally limited to operation on de-energized circuits, and conventional switching devices as described above must first be operated to de-energize a line before the elbow connector can be operated.
Since these conventional switches can be in locations remote from the particular elbow connector, a time consuming and elaborate procedure must be followed before the "pulling" of an elbow connector is permitted. These procedures involve considerable travel between points remote from each other. The time and distances of perhaps several miles introduce the hazard of inadvertant error or misoperation. As a consequence, all of this difficulty greatly extends the time that a circuit must be de-energized before a switching operation can be safely carried out.
In view of these problems, there is a need for a medium voltage load-break switch that is of small size and weight so that it can be used without special foundations and vaults, does not require cable loops, uses an insulating medium that does not present a fire hazard, has a long life, required substantially no maintenance, can be removely monitored, and is easy to de-energize.